The present invention relates to a radio connection system for connecting a base station and a mobile unit included in a digital mobile communication system using TDMA (Time Division Multiple Access) communication channels and, more particularly, to a radio connection system feasible for dynamic channel assignment.
A digital mobile communication system using TDMA communication channels, e.g., a radio telephone system has a plurality of base stations and a plurality of mobile units connectable to the base stations over radio channels. In each radio zone, the TDMA channels may be implemented as two TDMA channels each having a particular frequency, i.e., an upgoing channel and a down-going channel. Alternatively, a single TDMA channel may have the frame thereof divided into an up-going channel and a down-going channel each having a plurality of time slots. This kind of channel configuration is generally referred to as a Ping-Pong type configuration. Further, to enhance the efficient use of frequencies in such two conventional implementations, nearby radio zones may share a single radio frequency while using particular time slots each.
It has been customary with a mobile communication system to connect a base station and a mobile unit by the following procedure. On receiving a connection request from a mobile unit, a base station informs the mobile unit of, among time slots which it does not use, a time slot determined to be unoccupied by carrier sensing and a radio frequency assigned thereto. Then, the base station awaits an answer from the mobile unit in the unoccupied time slot. The mobile unit determines whether or not the reported time slot is unoccupied by carrier sensing and, if it is unoccupied returns an answer to the base station in that time slot. The reason that the mobile unit also executes such a decision is that an interference wave or a jamming wave is likely to affect the mobile unit although it may be negligible at the location of the base station. When the mobile unit determines that the reported time slot is occupied, the base station does not receive an answer before a predetermined period of time elapses. To increase the connection ratio, it is a common practice for the base station to report to the mobile unit a plurality of usable candidate time slots beforehand or to select another usable time slot on the expiration of the predetermined period of time and reports it to the mobile unit.
However, the problem with the conventional approaches for a higher connection ratio is that complicated sequence control with strict timings is needed. Therefore, both the base unit and the mobile unit have complicated control in units or software control.
Assume a system wherein nearby radio zones share the same radio frequency and use respective time slots in order to enhance efficient use of frequencies. Then, the probability increases that the time slot determined to be unoccupied by the base unit will be determined to be occupied by the mobile unit when it is occupied by the adjoining radio zone. This rather lowers the connection ratio while simply increasing the frequency of the above-stated complicated sequence.
The Ping-Pong type system using the same frequency band for transmission and reception has the following drawback. Assume that mobile units communicating with respective base stations at adjoining radio frequencies approach each other. Then, if the base stations are not synchronous in frame to each other, a leakage of the electric wave being sent from one mobile unit interferes with the communication of the other mobile unit. At this instant, the level of the leakage is extremely low. Hence, when a time slot is to be reported from some base station to one mobile unit, the leakage is not received by the base station despite the existence of another mobile unit communicating with an adjoining base station near the above-mentioned unit, again lowering the connection ratio.
The channels to be used by base stations have customarily been fixedly assigned. Nowadays, a dynamic channel assignment system is under study which changes the frequency every time a connection is set up, as needed. The prerequisite with such dynamic channel assignment is that the control over radio connection be simple.